It is well known to those skilled in the art that breathing apparatuses such as fully closed-cycle breathing apparatuses and similar—e.g. the specific sub-genre known as fully closed-cycle underwater breathing apparatus (CCUBA) or alternatively known as “closed-circuit rebreathers” or “CCR”—offer distinct advantages over the more common open-circuit breathing apparatuses such as e.g. Self-Contained Underwater Breathing Apparatuses (SCUBA) and the like. It should be emphasised that even if the text herein may focus on closed-cycled breathing apparatuses for diving purposes the same or similar advantages applies mutatis mutandis for closed-cycled breathing apparatuses in general and other breathing apparatuses wherein the amount of oxygen in the breathing gas has to be controlled.
Advantages provided by closed-cycle breathing apparatuses and similar are e.g. reduced bubble noise, extremely high gas usage efficiency, and optimized breathing gas composition etc. These and other advantages of closed-cycled breathing apparatuses such as CCRs derive from the fact that the exhaled breathing gas is recycled, filtered of carbon dioxide, replenished with oxygen, and returned to the diver for breathing again. The reduced bubble noise and the increased gas efficiency of a CCR both result from the fundamental function of recycling the breathing gas. The optimized breathing gas composition results from the fact that the oxygen control system of a CCR maintains a substantially constant partial-pressure of oxygen (rather than a constant fraction of oxygen, as in conventional open-circuit breathing apparatuses such as SCUBA and the like).
The partial pressure of a gas is a function of the fraction of the gas multiplied by the ambient pressure. As a diver descends and the depth increases, the ambient pressure also increases. Thus, for a given fraction of oxygen, the partial pressure increases as the depth increases. If the oxygen partial pressure exceeds a certain threshold (approximately 1.4 bar) the high concentration of oxygen and the risk of hyperoxia-induced seizure and other “oxygen toxicity” symptoms is considered unsafe for the diver. For example, the maximum safe depth at which a diver can breathe a mixture containing 50% oxygen is about 18 meters. On the other hand, the lower the oxygen concentration, the greater the concentration of non-oxygen gas constituents, such as nitrogen or helium. It is these non-oxygen components of the breathing mixture that lead to problems of decompression sickness (DCS), also known as “the bends”, which include symptoms ranging from pain in the joints, to paralysis, to death. To maximize the amount of time that can be safely spent at any given depth, the non-oxygen portions of the breathing gas should be kept to a minimum; which means that the oxygen should be kept to its maximum safe limit at all points during the dive.
Thus, the advantage of CCR over conventional open-circuit SCUBA in terms of optimized breathing gas composition results from the fact that a CCR can maintain the maximum safe partial pressure of oxygen (PO2) throughout all depths of a dive, thereby minimizing the concentration of non-oxygen gas constituents—leading to increased allowed time at any give depth and/or reduced risk of DCS.
But this advantage comes at a cost. Whereas the breathing mixture for a conventional open-circuit SCUBA diver is fixed based on the composition of the gas in the supply cylinder, the breathing mixture in a CCR is dynamic. Although it is this dynamic mixture capability that affords the CCR one of its primary advantages, a failure of the oxygen control system can be extremely dangerous. A malfunction that allows the PO2 to get too high places the diver at risk of a hyperoxia-induced seizure, which would almost certainly cause the diver to drown. A malfunction that allows the PO2 to get too low may lead to hypoxic-induced blackout, causing the diver to drown and/or suffer severe brain damages. Therefore, perhaps the most critical aspect of any CCR design involves the reliability of the oxygen control system.
Most modern CCRs incorporate one or more electronic oxygen sensors that directly measure the PO2 of the breathing gas. Most such sensors involve a galvanic reaction that produces a voltage output that is proportional to the concentration of the oxygen exposed to the sensor. Electronic systems interpret the signals from the oxygen sensor(s) to control a valve connected to an oxygen supply. When the oxygen sensors detect a PO2 below a certain “setpoint” threshold, the valve is opened and a small amount of oxygen is injected into the breathing gas. The reliability of the oxygen sensors, therefore, is of paramount importance for ensuring a safe breathing gas mixture when using a CCR.
There are a number of ways that oxygen sensors—considered by most experienced CCR divers as the weakest link in the oxygen control system—can fail (i.e., provide false readings), e.g. due to faulty calibration, sensor failure or condensation etc.
In the exemplifying discussions that follow we will assume a commonly available galvanic oxygen sensor (essentially a fuel cell that produces voltage output in response to the PO2 level) that is widely in use in CCR apparatus. However, the following discussions apply mutatis mutandis to all sensors that produce an output signal proportional to PO2 or similar for any other gas.
Calibration
All galvanic oxygen sensors must be calibrated to ensure accurate readings. If a sensor falls out of proper calibration the electronic control system of a CCR will misinterpret the readings. A calibration process typically involves exposing the sensors to one or more known gas mixtures at a known ambient pressure, and deriving calibration constants to the electronic logic that interprets the sensor readings. Calibration is typically conducted manually or semi-automatically prior to the dive, but is sometimes only done periodically. Calibration constants can be recorded incorrectly if the calibration gas mixture deviates from expected (e.g., if the calibration process assumes a mixture of 100% oxygen when a contaminated calibration gas is actually only 80% oxygen), if the ambient pressure is not properly taken into account, if the sensor fails in certain ways as described below, and/or if the user performs the calibration process incorrectly. Attempts to mitigate these problems have included automated calibration routines as part of the standard pre-dive process, incorporation of ambient pressure sensors into the calibration process, and testing against threshold values intended to detect calibration errors.
Sensor Failure by Exhaustion or Similar
Galvanic oxygen sensors eventually fail either through exhaustion of their chemical reaction or other age related degradation of active sensing elements and/or from a host of other environmental and user-caused effects (e.g. abuse, improper use). In many cases, a sensor will simply fail to generate sufficient output voltage at the time of calibration, and will be identified. In other cases, however, a sensor can perform normally up to a certain point, but deviate significantly from linearity in output voltage once the oxygen concentration exceeds a certain value. For example, a sensor could perform normally up to an oxygen concentration of 1.1 bar partial pressure, but then fail to produce a correspondingly higher output voltage at higher oxygen concentrations. Because the calibration process of most CCR systems uses 100% oxygen at ambient pressure (i.e, 1 bar partial pressure) in a pre-dive calibration, the calibration process may appear to complete correctly, but the system may not be able to properly interpret readings when the sensor is exposed to oxygen partial pressures above 1 bar.
Sensor Failure by Condensation or Similar
Moreover, one of the most common modes of oxygen sensor failure involves condensation. The breathing gas in a CCR is humidified to near-saturation when the gas is exhaled from the diver's lungs. In most cases, ambient water temperature is cooler than body core temperature, so as the breathing gas is cooled in the CCR breathing loop, liquid condensation inevitably forms. As a consequence, the inside walls of the CCR breathing pathways are typically dripping wet with condensation after a short period of time. The total volume of condensate can exceed several tens of millilitres per hour of dive time. This condensation can affect the oxygen sensor and cause erroneous readings. It can also lead to premature failure of the sensor. In some circumstances, a thin film of condensate can form across the active sensing face of the oxygen sensor (frequently a metal mesh or hydrophobic membrane), trapping a tiny pocket of gas against the sensor that is isolated from the breathing gas mixture. This is among the most dangerous forms of oxygen sensor failure, because it provides a false but plausible reading to the electronics, concealing the nature of the failure. For example, if the trapped pocket of gas has an oxygen concentration that is below a certain “setpoint” threshold, then the control system will continue to add oxygen to the breathing loop until the actual breathed PO2 reaches dangerously high levels. Conversely, if the trapped pocket of gas has a PO2 above the “setpoint” value, the control system will fail to add any oxygen at all, and the PO2 of the breathing gas will gradually diminish due to the diver's metabolic oxygen consumption, until hypoxic levels are reached and the diver blacks out.
Attempts to mitigate this problem include “water traps” and absorbent pads in the breathing loop designed to divert collected condensate away from the oxygen sensors; strategic placement of sensors in areas least likely to form condensation; placement of sensors on different planes to reduce the probability of multiple sensors collecting condensate simultaneously.
Even more importantly, almost all electronically-controlled CCR systems thus far developed attempt to safeguard against the consequences of failed oxygen sensors through the incorporation of triplex redundancy (that is, by incorporating three oxygen sensors in the CCR). This is i.a. based on the notion that if only one oxygen sensor is used, and it fails in a way that gives otherwise plausible readings, then there is no logical way to recognize that the sensor has failed. Similarly, if two sensors are used and one of them is giving a false reading, the control system can logically recognize a problem (unless both sensors fail in the same way), but cannot determine which sensor is correct and which has failed. With three oxygen sensors, so the conventional thinking goes, the system has “voting” logic. Assuming only one sensor fails at a time then the control system can be designed to interpret the two readings that agree within some pre-accepted tolerance as correct and thereby isolate the bad sensor reading.
Examples of breathing apparatuses that use three oxygen sensors and a “voting” logic or similar are disclosed in the patent documents U.S. Pat. No. 6,712,071 (Parker), GB 2404593 (Deas) and CA 2564999 (Straw).
Though ubiquitous among modern rebreather designs, the three-sensor approach to monitoring oxygen concentration in the breathing mixture is far from perfect. First, some more or less arbitrary threshold values must be established in order to carry out the voting logic. Because sensor readings can be slightly unstable in the chaotic breathing gas mixture of a CCR, a sensor must deviate from the other two sensors by a certain minimum threshold amount before it is considered suspect.
Then there is the question of what this threshold is measured against? For example, should the basis for the threshold comparison of one potentially errant sensor reading be the average value of the remaining two sensors, or the value of the sensor with the closest reading (i.e., the sensor giving the “middle” reading of the three) or perhaps something else?
Another problem with reliance upon the triple-redundant oxygen sensor system is the fact that sometimes two sensors fail the same way—often due to asymmetric condensate formation or because a user may have replaced one sensor with a fresh one and the other two are at the end of their useful life but may have exhibited in-range readings prior to the start of a dive (there are many such possibilities)—such that the apparently errant sensor reading is actually the correct reading. This mode of failure is particularly dangerous in that the control system actively ignores the true reading. Although this failure mode may seem unlikely, it has been documented on countless occasions in actual dive logs. Indeed, there have even been documented cases where all three sensors fail simultaneously such that all three give the same, but false reading. Other documented cases involve situations where no two sensors agree.
Once the threshold values and basis of comparison (voting logic algorithm) are determined, there is still the question of how best to adjust the oxygen control system in the event of an apparently failed sensor. Given two concordant values, and one errant value, should the control system simply ignore the errant value altogether and base its control logic on the average of the remaining two sensors? Or, should it base its control on the “middle” value of the three sensor readings—just in case the apparently errant sensor may be correct? Or, should additional logic be used such that the “setpoint” is adjusted dynamically, so that both the highest sensor value and the lowest sensor value are both kept within life-sustaining limits at all times? And what should the control system do in the even that no two oxygen sensors agree? Should it bias its logic to safeguard more rigorously against hypoxia, or hyperoxia?
Indeed, there are probably as many different answers to the questions and problems indicated above as there are people who have designed CCR oxygen control systems.
Although using three oxygen sensors and using sensors designed specifically for humid environments can mitigate some of the problems indicated above, all known CCR oxygen control systems are subject to failures due to one or more of the above problems. Hence, in view of the above there seems to be a need of improvements related to the control of oxygen in the breathing gas of a closed-cycle breathing apparatus and similar.